This invention relates to an antenna unit for receiving GPS signals transmitted from GPS satellites and, in particular, relates to an anti-slip structure thereof.
In recent years, the so-called global positioning system has been spreading in which a receiver receives signal waves transmitted respectively from a plurality of artificial satellites orbiting the earth and the current position of the receiver itself is detected based on information included in the received signal waves. This system is generally called a GPS (Global Positioning System) in those countries such as Japan and USA. The GPS generally uses GPS satellites controlled by the US Department of Defense. As similar systems, there are “GALILEO” used in Europe and “GLONASS” used in Russia. Herein, a positioning system using artificial satellites, the artificial satellites used in the positioning system, signal waves transmitted from the artificial satellites, receivers for receiving the signal waves, and so on are referred to as a GPS, GPS satellites, GPS signals, GPS receivers, and so on, respectively, for convenience sake.
The GPS is capable of detecting a current position of a GPS receiver itself with high accuracy and substantially in real time. Accordingly, the GPS is mainly used such that a GPS receiver is mounted in a moving object such as an automobile, an airplane, or a portable telephone and the current position of the moving object is measured.
Presently, GPS receivers that are suitable when installed in automobiles, i.e. so-called car GPS receivers, are rapidly spreading. When installing the GPS receiver in the automobile, a GPS receiving antenna unit for receiving GPS signals is disposed outside the automobile, for example, on a roof. Such a GPS receiving antenna unit is disclosed, for example, in Japanese Unexamined Patent Application Publication (JP-A) No. 2001-68912.
Referring to FIG. 1, a conventional antenna unit 100 will be described hereinbelow. The antenna unit 100 comprises an antenna case 103, an antenna module 104, a rubber packing member 105, and a signal line 106. The antenna case 103 is composed of a domed top cover 101 and a bottom plate 102 that are joined together. The antenna module 104 is disposed in the top cover 101. The packing member 105 is disposed at a joining portion between the top cover 101 and the bottom plate 102 to thereby ensure watertightness of the antenna case 103. The signal line 106 is connected to the antenna module 104.
The antenna module 104 comprises an antenna element 110 and a circuit board 111. The antenna element 110 is formed with an antenna for receiving GPS signals transmitted from GPS satellites. The circuit board 111 is formed with a circuit (hereinafter referred to as a “signal processing circuit”) adapted to perform various signal processing such as signal amplification with respect to a GPS signal received by the antenna element 110. The antenna element 110 and the circuit board 111 are bonded together by the use of a double-sided adhesive tape 112 or the like.
The signal line 106 is connected to the circuit board 111 for outputting the GPS signal to the outside of the antenna case 103. Further, a shield case 114 for shielding the signal processing circuit is attached to the circuit board 111 at its main surface on the side opposite to the side where the antenna element 110 is disposed. The signal line 106 is drawn out to the outside through a cutout portion 101a formed at the top cover 101. A gasket 115 is attached at a position corresponding to the cutout portion 101a. 
The antenna unit 100 is assembled by fixing the top cover 101 and the bottom plate 102 together by the use of four screws 120 in the state where the antenna module 104 and the packing member 105 are disposed in an inner space of the top cover 101.
The bottom plate 102 is formed with two concave portions 102a (only one of them is illustrated in FIG. 1). Permanent magnets (not shown) are disposed in the concave portions 102a, respectively. These permanent magnets are used for fixing by attraction the antenna unit 100 to a roof of an automobile. Further, a name plate 121 in the form of an aluminum thin plate indicating a type number, a name, etc. of the antenna unit 100 is disposed on a main surface of the bottom plate 102 on its side exposed outward. For the purpose of preventing damage to the roof of the automobile, a transparent resin sheet 122 is stuck to the main surface of the bottom plate 102 so as to cover the name plate 121 and substantially the whole of the main surface of the bottom plate 102.
As shown in FIG. 2, four projecting portions (legs) 105a are provided at a lower surface of the packing member 105. These projecting portions 105a pass through the bottom plate 102 and the resin sheet 122 so as to be exposed from a bottom surface of the antenna body. These projecting portions 105a serve to prevent slippage of the antenna body when it is placed on the roof of the automobile.
On the other hand, as shown in FIG. 3, the bottom plate 102 is formed with four through holes 102b for press-fitting the four projecting portions 105a therethrough. The diameter of each through hole 102b is substantially equal to that of each projecting portion 105a. The length of each projecting portion 105a is relatively long.
Therefore, as shown in FIG. 4, the packing member 105 is fixed to the bottom plate 102 by press-fitting the projecting portions 105a of the packing member 105 into the through holes 102b of the bottom plate 102.
As described above, in the conventional antenna unit 100, the packing member 105 is fixed to the bottom plate 102 by press-fitting the rubber legs 105a of the packing member 105 into the through holes 102b of the bottom plate 102. Therefore, in the conventional antenna unit 100, there is a problem that since each rubber leg 105a is relatively long and the diameters of each through hole 102b of the bottom plate 102 and each rubber leg 105a are substantially equal to each other, the operation performance is poor when press-fitting the soft rubber legs 105a into the through holes 102b of the bottom plate 102.